This NIH AREA proposal seeks to initiate studies to examine the processes that mediate the binding and entry of HIV-1. Although the critical role of the CD4 receptor is well-established, there is now increasing evidence that additional accessory molecules and co-receptors may also be involved in mediating the complex series of processes that comprise the early steps in infection. The recent discovery by Dr. Edward Berger and colleagues of the fusin co-receptor, a molecule that differentially mediates infection by laboratory-adapted viruses (vs monocytic primary isolates), provides the first direct evidence for a co-receptor of HIV infection, and the first link between cell tropism and the differential use of a cellular molecule. The present proposal seeks to delineate utilization of the fusin copreceptor along the continuum of HIV isolates (PIs) are resistant to recombinant HIV envelope (gp120) vaccine immunogens, and that adaption of a T-lymphocytropic (syncytium inducing) PI virus to persistent growth in T-cell lines (TCLs) renders the virus sensitive to neutralization by antisera to candidate rgp120 vaccines, as well as by specific monoclonal antibodies and soluble CD4. We identified genetic changes in the envelope gene, and in other as-yet unlocalized viral element(s), that mediate these concomitant changes in cell tropism and VN sensitivity. In collaboration with Dr. Berger, we will use these and other pedigreed PI and TCL-adapted viruses to define phenotypic correlates of fusin utilization. Monocytropic and non-syncytium inducing T-lymphocytropic PI viruses obtained from the same patients will also be assessed for use of the fusin co-receptor. Two complementary assays will be used to determine fusin utilization by these viruses: VN assays using antibodies directed against fusin will determine sensitivity to inhibition, and vaccinia virus-based fusogenic assays will determine the ability of molecularly cloned envelope genes to utilize this co-receptor. The preliminary studies and on-going collaboration enabled by this AREA grant will facilitate the principal investigator's transition to a grant-supported academic research career. A fuller understanding of the processes that mediate HIV binding and entry will suggest novel targets for the development of effective HIV vaccine immunogens and therapeutic antiviral interventions.